1. An atomic constant which enters into the formulas for wave numbers of atomic spectra, equal to 2π²me⁴/ch³, where m and e are the rest mass and charge of the electron, c is the speed of light, and h is Planck's constant; equal to 109,737.31534±0.00013 inverse centimeters. Symbolized R_∞.

2. For any atom, the Rydberg constant (first definition) divided by 1+m /M, where m and M are the masses of an electron and of the nucleus.